The present invention relates to a toner concentration control method for a developing device incorporated in a copier, printer, facsimile transceiver or similar image forming apparatus. More particularly, the present invention is concerned with a toner concentration control method for correcting, based on the output of an optical sensor representative of the density of a toner image for control formed on an image carrier, a target toner concentration for toner supplement control to be effected in response to the output of a toner concentration sensor which is mounted on the developing device.
A toner concentration control method of the kind described is disclosed in, for example, Japanese patent Laid-Open Publication Nos. 136667/1982 and 148679/1991. Generally, even when a toner is supplemented in response to the output of a toner concentration sensor to maintain a target toner concentration in the developing device, it is impossible to maintain the density of a toner image at desired one due to the aging of a photoconductive element or similar image carrier, developer, etc. The toner concentration control method, therefore, senses the density of image for control formed on the image carrier and corrects the target toner concentration such that the desired image density is set up. It has been customary to correct the target toner concentration once a day, i.e., when a power source is turned on, when an image forming operation is resumed after a certain period of time of suspension, or every time a predetermined number of copies are produced or an image forming operation is repeated over a predetermined period of time.
However, the conventional method simply increases or decreases the instantaneous target concentration by a predetermined amount in response to the sensed density of the toner image for control. This brings about a problem that the response to the changes in the characteristic of the image carrier and that of the developer due to aging is slow, making the image density unstable. Specifically, changes in developing characteristic which is susceptible to the aging of image carrier and developer depends on the environment in which an apparatus is operated. Therefore, the change in developing characteristic during the interval between consecutive corrections of target toner concentration differs from one apparatus to another. Assume that the density of the toner image for control is thinner than a target density, requiring the target toner concentration to be corrected to thicker one. Then, when the target toner concentration of that instant is simply increased by a predetermined amount, it may occur that, depending on the difference between the target concentration and the actual concentration in the developing device, toner supplement control using the corrected target concentration and effected in response to the output of the toner concentration sensor does not immediately determine that a supplement is necessary. In such a case, image formation will be executed with the toner concentration remaining low. Conversely, assume that the density of the toner image for control is thicker than the target density and requires the target concentration to be corrected to thinner one. Then, when the target concentration is simply reduced by the predetermined amount, it may occur that, depending on the above-mentioned difference, toner supply control using the corrected target concentration does not immediately determine that a supplement is not necessary, maintaining the toner concentration high.